Flame-retardant magnesium alloy and method of manufacturing same

ABSTRACT

A method of manufacturing a flame-retardant magnesium alloy having mechanical properties of a long period stacking ordered magnesium alloy and having an ignition temperature of 800° C. or more is provided. In the method of manufacturing a flame-retardant magnesium alloy, a flame-retardant magnesium alloy which contains a atomic % of Zn, b atomic % of at least one element selected from a group consisting of Gd, Tb, Tm and Lu, and x atomic % of Ca and in which a remaining part is formed of Mg and a, b and x satisfy Formulae 1 to 4 below is melted. 
       0.2≦ a ≦5.0  (Formula 1)
 
       0.5≦ b ≦5.0  (Formula 2)
 
       0.5 a −0.5≦ b   (Formula 3)
 
       0&lt; x ≦0.5  (Formula 4)

TECHNICAL FIELD

The present invention relates to a flame-retardant magnesium alloy and amethod of manufacturing the same.

BACKGROUND ART

A conventional long period stacking ordered (LPSO) magnesium alloy(refer to, for example, patent literatures 1 to 3) has mechanicalproperties of high strength and high ductility. The melting and castingtemperature of this long period stacking ordered magnesium alloy is 750°C. Since this temperature is close to an ignition temperature, it isdangerous to perform the melting and casting in the air. Therefore, inperforming the melting and casting, there has been a necessity ofperforming an operation thereof under an atmosphere (for example, underan atmosphere of vacuum and an inert gas) in which the combustion of themagnesium alloy is prevented. This increases the cost. In particular,since SF₆ that is used as an inert gas has 23,900 times as high globalwarming potential as carbon dioxide, SF₆ is harmful to the environment,and thus the utilization needs to be prevented.

PRIOR ART DOCUMENTS

-   Patent literature 1: Japanese Patent No. 3905115-   Patent literature 2: Japanese Patent No. 3940154-   Patent literature 3: Japanese Patent No. 4139841

DISCLOSURE OF THE INVENTION Problems to be Solved

An aspect of the present invention has an object to provide aflame-retardant magnesium alloy having mechanical properties of a longperiod stacking ordered magnesium alloy and having an ignitiontemperature of 800° C. or more and a method of manufacturing the same.

Solution to the Problem

Hereinafter, various aspects of the present invention will be described.

[1] A method of manufacturing a flame-retardant magnesium alloy, themethod including the step of melting a flame-retardant magnesium alloywhich contains a atomic % of Zn, in total, b atomic % of at least oneelement selected from a group consisting of Gd, Tb, Tm and Lu, and xatomic % of Ca and in which a remaining part is formed of Mg and a, band x satisfy Formulae 1 to 4 below,

0.1≦a≦5.0  (Formula 1)

0.25≦b≦5.0  (Formula 2)

0.5a−0.5≦b  (Formula 3)

0<x≦0.5 (preferably, 0.1≦x≦0.5, and further preferably,0.15≦x≦0.5).  (Formula 4)

[2] A method of manufacturing a flame-retardant magnesium alloycomprising a step of melting a flame-retardant magnesium alloy whichcontains a atomic % of Zn, x atomic % of Ca, in total, b atomic % of atleast one element selected from a group consisting of Gd, Tb, Tm and Lu,and a residue of Mg, wherein a, b and x satisfy formulae 1 to 4 below,

0.1≦a≦3.0  (Formula 1)

0.25≦b≦5.0  (Formula 2)

2a−3≦b  (Formula 3)

0<x≦0.5 (preferably, 0.1≦x≦0.5, and further preferably,0.15≦x≦0.5).  (Formula 4)

[3] The method of manufacturing a flame-retardant magnesium alloy in [1]above,

-   -   wherein a flame-retardant magnesium alloy in which said a and b        satisfy Formulae 1′ and 2′ below is melted,

0.2≦a≦5.0  (Formula 1′)

0.5≦b≦5.0.  (Formula 2′)

[4] The method of manufacturing a flame-retardant magnesium alloy in [2]above,

-   -   wherein a flame-retardant magnesium alloy in which said a and b        satisfy Formulae 1′ and 2′ below is melted,

0.2≦a≦3.0  (Formula 1′)

0.5≦b≦5.0.  (Formula 2′)

[5] The method of manufacturing a flame-retardant magnesium alloy in anyone of [1] to [4] above,

-   -   wherein said flame-retardant magnesium alloy has an ignition        temperature of 800° C. or more (preferably, 850° C. or more).

[6] The method of manufacturing a flame-retardant magnesium alloy in anyone of [1] to [5] above,

-   -   wherein said flame-retardant magnesium alloy is melted at a        temperature of 800° C. or less (preferably, 850° C. or less).

[7] The method of manufacturing a flame-retardant magnesium alloy in anyone of [1] to [6] above,

-   -   wherein said flame-retardant magnesium alloy is melted, and then        the melted flame-retardant magnesium alloy is cast.

[8] The method of manufacturing a flame-retardant magnesium alloy in [7]above,

-   -   wherein a cooling rate in casting said flame-retardant magnesium        alloy is 1000K/second or less (preferably, 100K/second or less).

[9] The method of manufacturing a flame-retardant magnesium alloy in anyone of [1] to [8],

-   -   wherein said flame-retardant magnesium alloy contains y atomic %        of Al, and y satisfies Formula 5 below,

0<y≦0.5 (preferably, 0.05≦y≦0.5).  (Formula 5)

[10] The method of manufacturing a flame-retardant magnesium alloy inany one of [3], [4] and [9] above,

-   -   wherein said flame-retardant magnesium alloy contains, in total,        c atomic % of at least one element selected from a group        consisting of La, Ce, Pr, Eu and Mm, and c satisfies Formula 6        and Formula 7 below,

0≦c≦2.0  (Formula 6)

0.5≦b+c≦6.0.  (Formula 7)

[11] The method of manufacturing a flame-retardant magnesium alloy inany one of [1], [2] and [9] above,

-   -   wherein said flame-retardant magnesium alloy contains, in total,        c atomic % of at least one element selected from a group        consisting of La, Ce, Pr, Eu and Mm, and c satisfies Formula 6        and Formula 7 below,

0≦c≦2.0  (Formula 6)

0.25≦b+c≦6.0.  (Formula 7)

[12] The method of manufacturing a flame-retardant magnesium alloy inany one of [3], [4] and [9] above,

-   -   wherein said flame-retardant magnesium alloy contains, in total,        c atomic % of at least one element selected from a group        consisting of Yb, Sm and Nd, and c satisfies Formula 6 and        Formula 7 below,

0≦c≦3.0  (Formula 6)

0.5≦b+c≦6.0.  (Formula 7)

[13] The method of manufacturing a flame-retardant magnesium alloy inany one of [1], [2] and [9] above,

-   -   wherein said flame-retardant magnesium alloy contains, in total,        c atomic % of at least one element selected from a group        consisting of Yb, Sm and Nd, and c satisfies Formula 6 and        Formula 7 below,

0≦c≦3.0  (Formula 6)

0.25≦b+c≦6.0.  (Formula 7)

[14] The method of manufacturing a flame-retardant magnesium alloy inany one of [3], [4] and [9] above,

-   -   wherein said flame-retardant magnesium alloy contains, in total,        c atomic % of at least one element selected from a group        consisting of Yb, Sm and Nd, contains, in total, d atomic % of        at least one element selected from a group consisting of La, Ce,        Pr, Eu and Mm, and c and d satisfy Formulae 6 to 8 below,

0≦c≦3.0  (Formula 6)

0≦d≦2.0  (Formula 7)

0.5≦b+c+d≦6.0.  (Formula 8)

[15] The method of manufacturing a flame-retardant magnesium alloy inany one of [1], [2] and [9] above,

-   -   wherein said flame-retardant magnesium alloy contains, in total,        c atomic % of at least one element selected from a group        consisting of Yb, Sm and Nd, contains, in total, d atomic % of        at least one element selected from a group consisting of La, Ce,        Pr, Eu and Mm, and c and d satisfy Formulae 6 to 8 below,

0≦c≦3.0  (Formula 6)

0≦d≦2.0  (Formula 7)

0.25≦b+c+d≦6.0.  (Formula 8)

[16] The method of manufacturing a flame-retardant magnesium alloy inany one of [1] to [15] above,

-   -   wherein said flame-retardant magnesium alloy contains, in total,        more than 0 atomic % and not more than 2.5 atomic % of at least        one element selected from a group consisting of Th, Si, Mn, Zr,        Ti, Hf, Nb, Ag, Sr, Sc, B, C, Sn, Au, Ba, Ge, Bi, Ga, In, Ir,        Li, Pd, Sb and V.

[17] A flame-retardant magnesium alloy, including an alloy whichcontains a atomic % of Zn, in total, b atomic % of at least one elementselected from a group consisting of Gd, Tb, Tm and Lu, and x atomic % ofCa, in which a remaining part is formed of Mg and a, b and x satisfyFormulae 1 to 4 below, and which includes a crystalline structure havinga long period stacking ordered structural phase.

0.1≦a≦5.0  (Formula 1)

0.25≦b≦5.0  (Formula 2)

0.5a−0.5≦b  (Formula 3)

0<x≦0.5 (preferably, 0.1≦x≦0.5, and further preferably,0.15≦x≦0.5)  (Formula 4)

[18] A flame-retardant magnesium alloy comprising a atomic % of Zn, xatomic % of Ca, in total, b atomic % of at least one element selectedfrom a group consisting of Gd, Tb, Tm and Lu, and a residue of Mg,

-   -   wherein a, b and x satisfy Formulae 1 to 4 below, and said alloy        comprises a crystalline structure having a long period stacking        ordered structural phase,

0.1≦a≦3.0  (Formula 1)

0.25≦b≦5.0  (Formula 2)

2a−3≦b  (Formula 3)

0<x≦0.5 (preferably, 0.1≦x≦0.5, and further preferably,0.15≦x≦0.5).  (Formula 4)

[19] The flame-retardant magnesium alloy in [17] above,

-   -   wherein a flame-retardant magnesium alloy in which said a and b        satisfy Formulae 1′ and 2′ below is melted,

0.2≦a≦5.0  (Formula 1′)

0.5≦b≦5.0.  (Formula 2′)

[20] The flame-retardant magnesium alloy in [18] above,

-   -   wherein a flame-retardant magnesium alloy in which said a and b        satisfy Formulae 1′ and 2′ below is melted,

0.2≦a≦3.0  (Formula 1′)

0.5≦b≦5.0.  (Formula 2′)

[21] The flame-retardant magnesium alloy in any one of [17] to [20]above,

-   -   wherein said alloy has an ignition temperature of 800° C. or        more (preferably, 850° C. or more).

[22] The flame-retardant magnesium alloy in any one of [17] to [21]above,

-   -   wherein said alloy contains y atomic % of Al, and y satisfies        Formula 5 below,

0<y≦0.5 (preferably, 0.05≦y≦0.5).  (Formula 5)

[23] The flame-retardant magnesium alloy in any one of [19], [20] and[22] above,

-   -   wherein said alloy contains, in total, c atomic % of at least        one element selected from a group consisting of La, Ce, Pr, Eu        and Mm, and c satisfies Formula 6 and Formula 7 below,

0≦c≦2.0  (Formula 6)

0.5≦b+c≦6.0.  (Formula 7)

[24] The flame-retardant magnesium alloy in any one of [17], [18] and[22] above,

-   -   wherein said alloy contains, in total, c atomic % of at least        one element selected from a group consisting of La, Ce, Pr, Eu        and Mm, and c satisfies Formula 6 and Formula 7 below,

0≦c≦2.0  (Formula 6)

0.25≦b+c≦6.0.  (Formula 7)

[25] The flame-retardant magnesium alloy in any one of [19], [20] and[22] above,

-   -   wherein said alloy contains, in total, c atomic % of at least        one element selected from a group consisting of Yb, Sm and Nd,        and c satisfies Formula 6 and Formula 7 below,

0≦c≦3.0  (Formula 6)

0.5≦b+c≦6.0.  (Formula 7)

[26] The flame-retardant magnesium alloy in any one of [17], [18] and[22] above,

-   -   wherein said alloy contains, in total, c atomic % of at least        one element selected from a group consisting of Yb, Sm and Nd,        and c satisfies Formula 6 and Formula 7 below,

0≦c≦3.0  (Formula 6)

0.25≦b+c≦6.0.  (Formula 7)

[27] The flame-retardant magnesium alloy in any one of [19], [20] and[22] above,

-   -   wherein said alloy contains, in total, c atomic % of at least        one element selected from a group consisting of Yb, Sm and Nd,        contains, in total, d atomic % of at least one element selected        from a group consisting of La, Ce, Pr, Eu and Mm, and c and d        satisfy Formulae 6 to 8 below,

0≦c≦3.0  (Formula 6)

0≦d≦2.0  (Formula 7)

0.5≦b+c+d≦6.0.  (Formula 8)

[28] The flame-retardant magnesium alloy in any one of [17], [18] and[22] above,

-   -   wherein said alloy contains, in total, c atomic % of at least        one element selected from a group consisting of Yb, Sm and Nd,        contains, in total, d atomic % of at least one element selected        from a group consisting of La, Ce, Pr, Eu and Mm, and c and d        satisfy Formulae 6 to 8 below,

0≦c≦3.0  (Formula 6)

0≦d≦2.0  (Formula 7)

0.25≦b+c+d≦6.0.  (Formula 8)

[29] The flame-retardant magnesium alloy in any one of [17] to [28]above,

-   -   wherein said alloy contains, in total, more than 0 atomic % and        not more than 2.5 atomic % of at least one element selected from        a group consisting of Th, Si, Mn, Zr, Ti, Hf, Nb, Ag, Sr, Sc, B,        C, Sn, Au, Ba, Ge, Bi, Ga, In, Ir, Li, Pd, Sb and V.

[30] The flame-retardant magnesium alloy in any one of [17] to [29]above,

-   -   wherein said alloy is a cast.

Effects of the Invention

An aspect of the present invention is applied, and thus it is possibleto provide a flame-retardant magnesium alloy having mechanicalproperties of a long period stacking ordered magnesium alloy and havingan ignition temperature of 800° C. or more and a method of manufacturingthe same.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph showing a relationship between the content of Ca,tensile yield strength and elongation when a tensile test is performedon a sample in an Example at room;

FIG. 2 is a graph showing a relationship between the content of Ca,tensile yield strength and elongation when a tensile test is performedon the sample in the Example at a temperature of 523K;

FIG. 3 is a graph showing a relationship between the content of Ca andan ignition temperature on the sample in the Example;

FIG. 4 is a SEM photograph showing the crystalline structure of anextrusion member of an alloy ofMg_(95.75-X)Zn₂Y_(1.9)La_(0.1)Al_(0.25)Ca_(X) (where X=0, 0.3, 0.6 and0.9) in the Example;

FIG. 5 is a SEM photograph and an EDS image showing the crystallinestructure of the extrusion member of an alloy ofMg_(95.75-X)Zn₂Y_(1.9)La_(0.1)Al_(0.25)Ca_(X) (where X=0.9) in theExample;

FIG. 6 is a graph showing a relationship between the content of Al,tensile yield strength and elongation when a tensile test is performedon a sample in a Comparative Example at room temperature;

FIG. 7 is a graph showing a relationship between the content of Al,tensile strength and elongation when a tensile test is performed on thesample in the Comparative Example at a temperature of 523K;

FIG. 8 is a SEM photograph showing the crystalline structure of anextrusion member of an alloy of Mg_(96-X)Zn₂Y_(1.9)La_(0.1)Al_(X) (whereX=0.05, 0.1, 0.15, 0.2 and 0.25) in the Comparative Example;

FIG. 9 is a SEM photograph showing the crystalline structure of anextrusion member of an alloy of Mg_(96-X)Zn₂Y_(1.9)La_(0.1)Al_(X) (whereX=0.3, 0.35, 0.4 and 0.5) in the Comparative Example;

FIG. 10 is an EDS image showing an extrusion member of an alloy ofMg_(95.7)Zn₂Y_(1.9)La_(0.1)Al_(0.3) in the Comparative Example;

FIG. 11 is an EDS image showing an extrusion member of an alloy ofMg_(95.5)Zn₂Y_(1.9)La_(0.1)Al_(0.5) in the Comparative Example;

FIG. 12 is a photograph showing the crystalline structure of anextrusion member of Mg_(96-X)Zn₂Y_(1.9)La_(0.1)Al_(X) (where X=0.1, 0.2,0.3, 0.4 and 0.5) in the Comparative Example;

FIG. 13 is a graph showing the results of a creep test on the extrusionmember of the Comparative Example; and

FIG. 14 is a photograph showing the crystalline X=0.25, 0.5, 1.0, 1.5and 2.0).

BEST MODE FOR CARRYING OUT THE INVENTION

Hereinafter, embodiments and examples of the present invention will beexplained in detail using the drawings.

However, a person skilled in the art would be able to easily understandthat the present invention is not limited to the following explanationbut the configuration and details thereof can be changed variouslywithout deviating from the gist and the scope of the present invention.Accordingly, the present invention should not be construed as beinglimited to the description of the present embodiments shown below.

With respect to the conditions such as the composition range and themanufacturing process, and the like for generating a long periodstacking ordered structural phase in a magnesium alloy according to eachof the embodiments described below, the reasons why the upper limit andthe lower limit of each of the components are determined and the reasonswhy the range of each of the conditions of the manufacturing process aredetermined, are as described in Japanese Patent No. 3905115, JapanesePatent No. 3940154 and Japanese Patent No. 4139841.

First Embodiment

A method of manufacturing a flame-retardant magnesium alloy according toan aspect of the present invention will be described.

An alloy which contains a atomic % of Zn, b atomic % of Y and x atomic %of Ca and in which the remaining part is formed of Mg, and a, b and xsatisfy formulae 1 to 4 below is melted and cast at a temperature of800° C. or less (preferably 850° C. or less). Since this alloy has anignition temperature of 800° C. or more (preferably 850° C. or more) bycontaining Ca. In this way, a magnesium alloy cast is made. The coolingrate at the time of casting is 1000K/second or less, and is morepreferably 100K/second or less.

0.5≦a<5.0  (Formula 1)

0.5<b<5.0  (Formula 2)

⅔a−⅚≦b  (Formula 3)

0<x≦0.5 (preferably, 0.1≦x≦0.5 and further preferably,0.15≦x≦0.5)  (Formula 4)

Various processes can be used as the process for producing the magnesiumalloy cast described above, and for example, high-pressure casting, rollcasting, inclined plate casting, continuous casting, thixomolding,die-casting and the like can be used. In addition, a product cut into apredetermined shape from an ingot may be used as the magnesium alloycast.

Then, homogenized heat treatment may be performed on the magnesium alloycast. Preferably, in the conditions of the heat treatment at this time,the temperature is set at 400 to 550° C., and the treatment time is setat 1 to 1500 minutes (or 24 hours).

Then, plastic processing is performed on the magnesium alloy cast.Examples of the methods of performing this plastic processing, that areused, include extrusion, an ECAE (equal-channel-angular-extrusion)processing method, rolling, drawing and forging, processing of repeatingthese, FSW (friction stir welding) processing and the like.

Preferably, when the plastic processing is performed by extrusion, theextrusion temperature is set to 250° C. or more and 500° C. or less, andthe cross-section reduction rate by extrusion is set to 5% or more.

The ECAE processing method is a method of rotating the longitudinaldirection of the sample by 90° per pass in order to introduce uniformdistortion into the sample. Specifically, the ECAE processing method isthe following method: a magnesium alloy cast serving as a moldingmaterial is forcibly made to enter a molding hole of a molding die wherethe molding hole whose cross section is in the shape of the letter L isformed, in particular, in a part of the L-shaped molding hole that isbent by 90°, a stress is applied to the magnesium alloy cast and thus amolded member having excellent strength and toughness is obtained. Thenumber of passes of the ECAE is preferably 1 to 8. The number is morepreferably 3 to 5. The temperature at the time of processing of the ECAEis preferably 250° C. or more and 500° C. or less

When the plastic processing is performed by rolling, it is preferablethat the rolling temperature is set to 250° C. or more and 500° C. orless, and the rolling reduction rate is set to 5% or more.

When the plastic processing is performed by drawing processing, it ispreferable that the temperature at which the drawing processing isperformed is set to 250° C. or more and 500° C. or less, and thecross-section reduction rate in the drawing processing is set to 5% ormore.

When the plastic processing is performed by forging, it is preferablethat the temperature at which the forging processing is performed is setto 250° C. or more and 500° C. or less, and the processing rate in theforging processing is set to 5% or more.

In the plastic processing performed on the magnesium alloy cast, it ispreferable that the amount of distortion for each processing is 0.002 ormore and 4.6 or less, and the total amount of distortion is 15 or less.In the plastic processing, it is preferable that the amount ofdistortion for each processing is 0.002 or more and 4.6 or less, and thetotal amount of distortion is 10 or less. The reason why the preferabletotal amount of distortion is set to 15 or less and the more preferabletotal amount of distortion is set to 10 or less is because even when thetotal amount of distortion is increased, the strength of the magnesiumalloy is not always increased accordingly and as the total amount ofdistortion is made larger, the manufacturing cost is increased.

Note that the amount of distortion in the ECAE processing is 0.95 to1.15/each processing, and for example, when 16 times of the ECAEprocessing are performed, the total amount of distortion is0.95×16=15.2, whereas when 8 times of the ECAE processing are performed,the total amount of distortion is 0.95×8=7.6.

In addition, the amount of distortion in the extrusion processing is0.92/each processing when the extrusion ratio is 2.5, is 1.39/eachprocessing when the extrusion ratio is 4, is 2.30/each processing whenthe extrusion ratio is 10, is 2.995/each processing when the extrusionratio is 20, is 3.91/each processing when the extrusion ratio is 50, is4.61/each processing when the extrusion ratio is 100 and is 6.90/eachprocessing when the extrusion ratio is 1000.

The plastic-processed product obtained by performing the plasticprocessing on the magnesium alloy cast as described above has acrystalline structure of an hcp structure magnesium phase and a longperiod stacking ordered structural phase at room temperature, the volumefraction of the crystal grains of the long period stacking orderedstructure is 5% or more (more preferably 10% or more) and the crystalgrain size of the magnesium alloy is 100 nm or more and 500 μm or less.The average grain size of the hcp structure magnesium phase is 2 μm ormore, and the average grain size of the long period stacking orderedstructural phase is 0.2 μm or more. Within the crystal grain of the longperiod stacking ordered structural phase, a plurality of random grainboundaries is present, and the average grain size of the crystal grainspecified by the random grain boundaries is 0.05 μm or more. Thedislocation density is high in the random grain boundary, but thedislocation density is low in the portions of the long period stackingordered structural phase other than the random grain boundaries.Therefore, the dislocation density of the hcp structure magnesium phaseis one or more digits larger than the dislocation density in theportions of the long period stacking ordered structural phase other thanthe random grain boundaries.

At least part of the long period stacking ordered structural phase iscurved or bent. The plastic-processed product may have at least one typeof precipitate selected from a precipitate group consisting of acompound of Mg and a rare-earth element, a compound of Mg and Zn, acompound of Zn and a rare-earth element, and a compound of Mg and Zn anda rare-earth element. The total volume fraction of the precipitate ispreferably more than 0% and not more than 40%. Additionally, theplastic-processed product has hcp-Mg. In a plastic-processed productafter the plastic processing is performed, both the Vickers hardness andthe yield strength are increased as compared with a cast before theplastic processing is performed.

Heat treatment may be performed on the plastic-processed product afterthe plastic processing is performed on the magnesium alloy cast.Preferably, in the conditions of the heat treatment, the temperature isset to not less than 200° C. and less than 500° C., and the heattreatment time is set at 10 to 1500 minutes (or 24 hours). The reasonwhy the heat treatment temperature is set to less than 500° C. isbecause when the heat treatment temperature is set to 500° C. or more,the amount of distortion applied by the plastic processing is cancelled.

In the plastic-processed product after the heat treatment is performed,both the Vickers hardness and the yield strength are increased ascompared with the plastic-processed product before the heat treatment isperformed. Furthermore, the plastic-processed product after the heattreatment has, as with the plastic-processed product before the heattreatment, a crystalline structure of an hop structure magnesium phaseand a long period stacking ordered structural phase at room temperature,the volume fraction of the crystal grains of the long period stackingordered structure becomes 5% or more (more preferably 10% or more), theaverage grain diameter of the hcp structure magnesium phase is 2 μm ormore, and the average grain diameter of the long period stacking orderedstructural phase is 0.2 μm or more. Within the crystal grain of the longperiod stacking ordered structural phase, a plurality of random grainboundaries is present, and the average grain diameter of the crystalgrain specified by the random grain boundaries is 0.05 μm or more. Thedislocation density is high in the random grain boundary, but thedislocation density is low in the portions of the long period stackingordered structural phase other than the random grain boundaries.Therefore, the dislocation density of the hcp structure magnesium phaseis one or more digits larger than the dislocation density in theportions of the long period stacking ordered structural phase other thanthe random grain boundaries.

At least a part of the long period stacking ordered structural phase ofthe plastic-processed product after the heat treatment is curved orbent. In addition, the plastic-processed product may have at least onetype of precipitate selected from a precipitate group consisting of acompound of Mg and a rare-earth element, a compound of Mg and Zn, acompound of Zn and a rare-earth element, and a compound of Mg and Zn anda rare-earth element. The total volume fraction of the precipitate ispreferably more than 0% and not more than 40%.

In the present embodiment, in the processes of melting and casting formanufacturing a magnesium alloy including mechanical properties of highstrength and high ductility by having the long period stacking orderedstructural phase, it becomes possible to carry out the processes in theair without setting an atmosphere in which combustion is prevented (aninert gas atmosphere having problems in cost and environment). Thereason for this is that it is possible to set the ignition temperatureof the magnesium alloy to 800° C. or more (preferably 850° C. or more)by addition of a small amount of Ca. The addition amount of Ca is 0atomic % or more and 0.5 atomic % or less (is preferably more than 0.1atomic % and not more than 0.5 atomic % and is further preferably 0.15atomic % or more and 0.5 atomic % or less).

Namely, when Ca is not added, since the temperature of the magnesiumalloy at the time of melting is close to the ignition temperature, it isnecessary to adopt an atmosphere in which combustion is prevented,whereas it is possible to make the ignition temperature be higher thanthe temperature at the time of melting by addition of a small amount ofCa, it becomes possible to perform the melting and casting in the air.

In addition, the magnesium alloy of the present embodiment is an alloyobtained by achieving flame retardance by the increase in the ignitiontemperature, conventional metal processing facilities may be utilizedwithout being changed, and it is possible to reduce risk of ignitingfine powder or cutting chips generated at the time of processing, withthe result that problems related to the environment, the cost and thesafety in the processing steps can be solved at the same time.

Additionally, the magnesium alloy according to the present embodimentcan increase the strength by having the long period stacking orderedstructural phase, and has a property less likely to be combusted at thetime of melting, casting and processing. Namely, it is possible torealize the magnesium alloy that is advantageous both in high strengthand flame retardance.

Furthermore, the range of the application of the magnesium alloy in thepresent embodiment covers various fields such as an IT field (a smartphone, a notebook computer and the like), a medical field, an automobilefield, an airplane field and a railway field.

Moreover, the range of the composition of the magnesium alloy in thepresent embodiment will be described below.

When the content of zinc is 5 atomic % or more, in particular, thetoughness (or the ductility) tends to be lowered. In addition, when thetotal content of Y is 5 atomic % or more, in particular, the toughness(or the ductility) tends to be lowered.

The increases in strength and toughness are remarkable when the contentof zinc is 0.5 to 1.5 atomic %. When the content of zinc is near 0.5atomic % and the content of a rare-earth element is decreased, thestrength tends to be lowered, but even in such a range, the strength andthe toughness are higher than those in a conventional case. Therefore,the range of the content of zinc in the magnesium alloy of the presentembodiment is at the widest range 0.5 atomic % or more and 5.0 atomic %or less.

Although the Mg—Zn—Y magnesium alloy of the present embodiment has thecontent in the range described above, impurities to the extent of notaffecting the properties of the alloy may be contained.

Note that the magnesium alloy of the present embodiment may furthercontain y atomic % of Al, and y satisfies formula 5 below, preferablysatisfies formula 51 below, further preferably satisfies formula 52 orformula 53 below and more preferably satisfies formula 54 or formula 55below. The upper limit of the content of Al is set to less than 0.35atomic % (preferably 0.3 atomic % or less), and thus it is possible tomaintain high strength at high temperature.

0<y≦0.5  (Formula 5)

0.05≦y≦0.5  (Formula 51)

0<y<0.35  (Formula 52)

0.05≦y<0.35  (Formula 53)

0<y≦0.3  (Formula 54)

0.05≦y≦0.3  (Formula 55)

In addition, the magnesium alloy of the present embodiment may contain,in total, c atomic % of at least one element selected from a groupconsisting of La, Ce, Pr, Eu, Mm and Gd, and c preferably satisfiesformula 6 and formula 7 below or formula 7 and formula 8 below.

0≦c<2.0  (Formula 6)

0.2≦b+c≦6.0  (Formula 7)

c/b≦1.5  (Formula 8)

Mm (misch metal) is a mixture or an alloy of a plurality of rare-earthelements each containing Ce and La as the main components, and is aresidue after Sm, Nd and the like which are useful rare-earth elementsare removed by being refined from ores, and its composition depends onthe compositions of the ores before refinement.

The main reason why the upper limit of the content of La and the like isset to 2.0 atomic % is because there is almost no solid solubility limitof La and the like. Additionally, the reason why La and the like arecontained is because an effect of miniaturizing crystal grains and aneffect of precipitating an intermetallic compound are obtained.

Furthermore, the magnesium alloy of the present embodiment may contain,in total, c atomic % of at least one element selected from a groupconsisting of Yb, Tb, Sm and Nd, and c preferably satisfies formula 8and formula 9 below.

0≦c≦3.0  (Formula 8)

0.2≦b+c≦6.0  (Formula 9)

The reason why the upper limit of the content of Yb and the like is setto 3.0 atomic % is because the solid solubility limit of Yb and the likeis low. In addition, the reason why Yb and the like are contained isbecause an effect of miniaturizing crystal grains and an effect ofprecipitating an intermetallic compound are obtained.

Moreover, the magnesium alloy of the present embodiment may contain, intotal, c atomic % of at least one element selected from a groupconsisting of Yb, Tb, Sm and Nd, may contain, in total, d atomic % of atleast one element selected from a group consisting of La, Ce, Pr, Eu, Mmand Gd, and c and d preferably satisfy formulae 6 to 8 or formulae 8 and9 below.

0≦c≦3.0  (Formula 6)

0≦d<2.0  (Formula 7)

0.2≦b+c+d≦6.0  (Formula 8)

d/b≦1.5  (Formula 9)

The reason why the total content of Y, Yb and the like and La and thelike is set to 6.0 atomic % or less is because, when the total contentexceeds 6%, the weight of the magnesium alloy is increased, the cost ofraw materials is increased and furthermore, the toughness is lowered. Inaddition, the reason why Yb and the like and La and the like arecontained is because an effect of miniaturizing crystal grains and aneffect of precipitating an intermetallic compound are obtained.Additionally, the reason why formula 9 above is preferably satisfied isbecause when d/b is made greater than 1.5 times, the effect of formationof the long period stacking ordered structural phase is reduced and theweight of the magnesium alloy is increased.

Furthermore, the magnesium alloy of the present embodiment preferablycontains, in total, more than 0 atomic % and not more than 2.5 atomic %of at least one element selected from a group consisting of Th, Si, Mn,Zr, Ti, Hf, Nb, Ag, Sr, Sc, B, C, Sn, Au, Ba, Ge, Bi, Ga, In, Ir, Li,Pd, Sb and V. When Th and the like are added, it is possible to improvethe other qualities while maintaining high strength and high toughness.The addition of Th and the like is effective for, for example, corrosionresistance, grain miniaturization and the like.

Moreover, when the magnesium alloy obtained by adding more than 0 atomic% and not more than 2.5 atomic % of Zr is melted and cast, in themagnesium alloy cast, the precipitation of a compound such as Mg₃Zn₃RE₂is reduced, the formation of the long period stacking ordered structuralphase is facilitated and the crystalline structure is miniaturized.Therefore, plastic processing such as extrusion is easily performed onthis magnesium alloy cast, and the plastic-processed product on whichthe plastic processing is performed has a larger amount of long periodstacking ordered structural phases and a more miniaturized crystallinestructure than a plastic-processed product of the magnesium alloy towhich Zr is not added. The plastic-processed product has a large amountof long period stacking ordered structural phases, and thus it ispossible to enhance strength and toughness.

Second Embodiment

A method of manufacturing a flame-retardant magnesium alloy according toone aspect of the present invention will be described. Note that, in themethod of manufacturing a flame-retardant magnesium alloy according tothe second embodiment, the description of the same parts as in themethod of manufacturing a flame-retardant magnesium alloy according tothe first embodiment will be omitted as much as possible.

An alloy which contains a atomic % of Zn, b atomic % of Y and x atomic %of Ca and in which the remaining part is formed of Mg and a, b and xsatisfy formulae 1 to 4 below is melted and cast at a temperature of800° C. or less (preferably 850° C. or less). Since this alloy has anignition temperature of 800° C. or more (preferably 850° C. or more) bycontaining Ca. In this way, a magnesium alloy cast is formed. As themagnesium alloy cast, a product cut into a predetermined shape from aningot is used.

0.25≦a≦5.0  (Formula 1)

0.5≦b≦5.0  (Formula 2)

0.5a≦b  (Formula 3)

0<x≦0.5 (preferably, 0.1≦x≦0.5 and further preferably,0.15≦x≦0.5)  (Formula 4)

Next, a plurality of chip-shaped casts having a size of several mmsquare or less is produced by cutting the magnesium alloy cast.

Then, the chip-shaped casts may be preliminarily molded using acompression means or a means of a plastic processing method, and thusmay be subjected to homogenized heat treatment. Preferably, in theconditions of the heat treatment at this time, the temperature is set to400 to 550° C., and the treatment time is set to 1 to 1500 minutes (or24 hours). In addition, heat treatment may be performed on thepreliminarily molded product at a temperature of 150 to 450° C. for 1 to1500 minutes (or 24 hours).

The chip-shaped cast is generally used as, for example, the raw materialof thixotropic mold.

Note that a mixture of the chip-shaped cast and ceramic particles may bepreliminarily molded using a compression means or a means of a plasticprocessing method, and thus may be subjected to homogenized heattreatment. Furthermore, before the chip-shaped cast is preliminarilymolded, the cast may be additionally subjected to strong distortionprocessing.

Subsequently, plastic processing is performed on the chip-shaped cast,and thus the chip-shaped cast is solidified and molded. Various methodscan be used as the method of performing the plastic processing in thesame way as in the first embodiment. Note that, before the chip-shapedcast is solidified and molded, repetition processing including:mechanical alloying such as a ball mill, a stamp mill, or a high-energyball mill; or bulk mechanical alloying may be added. In addition, afterthe solidification and molding, plastic processing or blastingprocessing may further be added. Furthermore, the magnesium alloy castmay be complexed with intermetallic compound particles, ceramicparticles, fibers or the like, or the cut product may be mixed withceramic particles, fibers or the like.

The plastic-processed product obtained by performing the plasticprocessing as described above has, at room temperature, a crystallinestructure of the hcp structure magnesium phase and the long periodstacking ordered structural phase. At least a part of the long periodstacking ordered structural phase is curved or bent. With regard to theplastic-processed product after the plastic processing is performed,both the Vickers hardness and the yield strength are increased ascompared with the cast before the plastic processing is performed.

The total amount of distortion when the plastic processing is performedon the chip-shaped cast is preferably 15 or less, and is more preferably10 or less. The amount of distortion for each processing is preferably0.002 or more and 4.6 or less.

Note that the total amount of distortion here is the total amount ofdistortion that is not cancelled by heat treatment such as annealing,and means the total amount of distortion when the plastic processing isperformed after the chip-shaped cast is preliminarily molded. Namely,distortion that is cancelled by performing heat treatment in the middleof the manufacturing process is not counted as the total amount ofdistortion, and the amount of distortion until the chip-shaped cast ispreliminarily molded is not counted as the total amount of distortion.

Heat treatment may be performed on the plastic-processed product afterthe plastic processing is performed on the chip-shaped cast. Preferably,in the conditions of the heat treatment, the temperature is set to notless than 200° C. and less than 500° C., and the heat treatment time isset to 10 to 1500 minutes (or 24 hours). The reason why the heattreatment temperature is set to less than 500° C. is because, when theheat treatment temperature is set to 500° C. or more, the amount ofdistortion applied by the plastic processing is cancelled.

In the plastic-processed product after the heat treatment is performed,both the Vickers hardness and the yield strength are increased ascompared with the plastic-processed product before the heat treatment isperformed. In addition, the plastic-processed product after the heattreatment has, as with the plastic-processed product before the heattreatment, a crystalline structure of an hcp structure magnesium phaseand a long period stacking ordered structural phase at room temperature.At least a part of the long period stacking ordered structural phase iscurved or bent.

Also in the present embodiment, the same effects as in the firstembodiment can be obtained.

In addition, in the present embodiment, the chip-shaped cast is producedby cutting the cast, and thus the structure is miniaturized, with theresult that, as compared with the first embodiment, it becomes possibleto produce, for example, the plastic-processed product having highstrength, high ductility and high toughness. Additionally, in themagnesium alloy of the present embodiment, even when the concentrationsof zinc and a rare-earth element are lower than in the magnesium alloyof the first embodiment, it is possible to obtain the properties of highstrength and high toughness.

Furthermore, when the content of zinc is less than 0.25 atomic % or thecontent of Y is less than 0.5 atomic, at least one of strength andtoughness is insufficient. Therefore, the lower limit of the content ofzinc is set to 0.25 atomic, and the lower limit of the total content ofthe rare-earth element is set to 0.5 atomic %. The reason why the lowerlimit of the content of zinc can be as low as one half of that in thefirst embodiment is because the application to the chip-shaped cast iscarried out.

Moreover, although the Mg—Zn—Y magnesium alloy of the present embodimenthas the content in the range described above, impurities to the extentof not affecting the properties of the alloy may be contained.

Note that the magnesium alloy of the present embodiment may contain, intotal, c atomic % of at least one element selected from a groupconsisting of La, Ce, Pr, Eu, Mm and Gd, and c preferably satisfiesformula 6 and formula 7 below.

0≦c≦3.0  (Formula 6)

0.1≦b+c≦6.0  (Formula 7)

In addition, the magnesium alloy of the present embodiment may contain,in total, c atomic % of at least one element selected from a groupconsisting of Yb, Tb, Sm and Nd, and c preferably satisfies formula 8and formula 9 below.

0≦c≦3.0  (Formula 8)

0.1≦b+c≦6.0  (Formula 9)

Additionally, the magnesium alloy of the present embodiment may contain,in total, c atomic % of at least one element selected from a groupconsisting of Yb, Tb, Sm and Nd, may contain, in total, d atomic % of atleast one element selected from a group consisting of La, Ce, Pr, Eu, Mmand Gd, and c and d preferably satisfy formulae 6 to 8 below.

0≦c≦3.0  (Formula 6)

0≦d≦3.0  (Formula 7)

0.1≦b+c+d≦6.0  (Formula 8)

Third Embodiment

A method of manufacturing a flame-retardant magnesium alloy according toone aspect of the present invention will be described. Note that, in themethod of manufacturing a flame-retardant magnesium alloy according tothe third embodiment, the description of the same parts as in the methodof manufacturing a flame-retardant magnesium alloy according to thefirst embodiment will be omitted as much as possible.

A flame-retardant magnesium alloy which contains a atomic % of Zn, intotal, b atomic % of at least one element selected from a groupconsisting of Gd, Tb, Tm and Lu, and x atomic % of Ca and in which aremaining part is formed of Mg and a, b and x satisfy Formulae 1 to 4below is melted at a temperature of 800° C. or less (preferably, 850° C.or less) and cast. This alloy has an ignition temperature of 800° C. ormore (preferably 850° C. or more) by containing Ca. In this way, amagnesium alloy cast is made. The cooling rate at the time of casting is1000K/second or less, more preferably 100K/second or less.

0.2≦a≦5.0  (Formula 1)

0.5≦b≦5.0  (Formula 2)

0.5a−0.5≦b  (Formula 3)

0<x≦0.5 (preferably, 0.1≦x≦0.5, and further preferably,0.15≦x≦0.5)  (Formula 4)

Next, homogenized heat treatment may be performed on the magnesium alloycast.

Then, plastic processing is performed on the magnesium alloy cast.

The plastic-processed product obtained by performing the plasticprocessing on the magnesium alloy cast as described above has acrystalline structure of an hcp structure magnesium phase and a longperiod stacking ordered structural phase at room temperature, the volumefraction of the crystal grains of the long period stacking orderedstructure is 5% or more (more preferably 10% or more).

At least a part of the long period stacking ordered structural phase iscurved or bent.

Heat treatment may be performed on the plastic-processed productobtained by performing the plastic processing on the magnesium alloycast. Preferably, in the conditions of the heat treatment at this time,the temperature is set to 400 to 550° C., and the heat treatment time isset to 1 to 1500 minutes (or 24 hours).

In the plastic-processed product after the heat treatment is performed,both the Vickers hardness and the yield strength are increased ascompared with the plastic-processed product before the heat treatment isperformed. Furthermore, the plastic-processed product after the heattreatment has, as with the plastic-processed product before the heattreatment, a crystalline structure of an hcp structure magnesium phaseand a long period stacking ordered structural phase at normaltemperature.

Also in the present embodiment, the same effects as in the firstembodiment can be obtained.

Moreover, although the Mg—Zn—(Gd, Tb, Tm, Lu)-based magnesium alloy ofthe present embodiment has the content in the range described above,impurities to the extent of not affecting the properties of the alloymay be contained.

Note that the magnesium alloy of the present embodiment may furthercontain y atomic % of Al, and y satisfies Formula 5 below, preferablysatisfies Formula 51 below, further preferably satisfies Formula 52 orFormula 53 below, and more preferably satisfies Formula 54 or Formula 55below. The strength at a high temperature can be maintained at a highlevel by setting the upper limit of the content of Al to less than 0.35atomic % (preferably 0.3 atomic % or less).

0<y≦0.5  (Formula 5)

0.05≦y≦0.5  (Formula 51)

0<y<0.35  (Formula 52)

0.05≦y<0.35  (Formula 53)

0<y≦0.3  (Formula 54)

0.05≦y≦0.3  (Formula 55)

Note that the magnesium alloy of the present embodiment may contain, intotal, c atomic % of at least one element selected from a groupconsisting of La, Ce, Pr, Eu and Mm, and c preferably satisfies Formulae6 and 7 below.

0≦c≦2.0  (Formula 6)

0.5≦b+c≦6.0  (Formula 7)

In addition, the magnesium alloy of the present embodiment may contain,in total, c atomic % of at least one element selected from a groupconsisting of Yb, Sm and Nd, and c preferably satisfies Formulae 6 and 7below.

0≦c≦3.0  (Formula 6)

0.5≦b+c≦6.0  (Formula 7)

Furthermore, the magnesium alloy of the present embodiment may contain,in total, c atomic % of at least one element selected from a groupconsisting of Yb, Sm and Nd, may contain, in total, d atomic % of atleast one element selected from a group consisting of La, Ce, Pr, Eu andMm, and c and d satisfy Formulae 6 to 8 below.

0≦c≦3.0  (Formula 6)

0≦d≦2.0  (Formula 7)

0.5≦b+c+d≦6.0  (Formula 8)

Moreover, the magnesium alloy of the present embodiment preferablycontains, in total, more than 0 atomic % and not more than 2.5 atomic %of at least one element selected from a group consisting of Th, Si, Mn,Zr, Ti, Hf, Nb, Ag, Sr, Sc, B, C, Sn, Au, Ba, Ge, Bi, Ga, In, Ir, Li,Pd, Sb and V. When Th and the like are added, it is possible to improvethe other qualities while maintaining high strength and high toughness.The addition of Th and the like is effective for, for example, corrosionresistance, grain miniaturization and the like.

In addition, when the magnesium alloy obtained by adding more than 0atomic % and not more than 2.5 atomic % of Zr is melted and cast, in themagnesium alloy cast, the precipitation of a compound such as Mg₃Zn₃RE₂is suppressed, the formation of the long period stacking orderedstructural phase is facilitated, and the crystalline structure isminiaturized. Accordingly, plastic processing such as extrusion iseasily performed on this magnesium alloy cast, and the plastic-processedproduct on which the plastic processing is performed has a larger amountof long period stacking ordered structural phases and a moreminiaturized crystalline structure than a plastic-processed product ofthe magnesium alloy to which Zr is not added. In this way, theplastic-processed product has a large amount of the long period stackingordered structural phases, and thus it is possible to enhance strengthand toughness.

Fourth Embodiment

A method of manufacturing a flame-retardant magnesium alloy according toone aspect of the present invention will be described. Note that, in themethod of manufacturing a flame-retardant magnesium alloy according tothe fourth embodiment, the description of the same parts as in themethod of manufacturing a flame-retardant magnesium alloy according tothe first embodiment will be omitted as much as possible. Aflame-retardant magnesium alloy which contains a atomic % of Zn, intotal, b atomic % of at least one element selected from a groupconsisting of Gd, Tb, Tm and Lu, and x atomic % of Ca and in which aremaining part is formed of Mg and a, b and x satisfy Formulae 1 to 4below is melted at a temperature of 800° C. or less (preferably, 850° C.or less) and cast. This alloy has an ignition temperature of 800° C. ormore (preferably 850° C. or more) by containing Ca. In this way, amagnesium alloy cast is made. The cooling rate at the time of casting is1000K/second or less, more preferably 100K/second or less.

0.2≦a≦3.0  (Formula 1)

0.5≦b≦5.0  (Formula 2)

2a−3≦b  (Formula 3)

0<x≦0.5 (preferably, 0.1≦x≦0.5, and further preferably,0.15≦x≦0.5)  (Formula 4)

The process of making the above magnesium alloy cast and the subsequentprocesses are the same as those in the third embodiment.

Also according to the present embodiment, the same effects can beobtained as in the third embodiment.

Fifth Embodiment

A method of manufacturing a flame-retardant magnesium alloy according toone aspect of the present invention will be described. Note that, in themethod of manufacturing a flame-retardant magnesium alloy according tothe fifth embodiment, the description of the same parts as in the methodof manufacturing a flame-retardant magnesium alloy according to thethird embodiment will be omitted as much as possible.

A flame-retardant magnesium alloy which contains a atomic % of Zn, intotal, b atomic % of at least one element selected from a groupconsisting of Gd, Tb, Tm and Lu, and x atomic % of Ca and in which aremaining part is formed of Mg and a, b and x satisfy Formulae 1 to 4below is melted at a temperature of 800° C. or less (preferably, 850° C.or less) and cast. This alloy has an ignition temperature of 800° C. ormore (preferably 850° C. or more) by containing Ca. In this way, amagnesium alloy cast is made. A product cut into a predetermined shapefrom an ingot is used as the magnesium alloy cast.

0.1≦a≦5.0  (Formula 1)

0.25≦b≦5.0  (Formula 2)

0.5a−0.5≦b  (Formula 3)

0<x≦0.5 (preferably, 0.1≦x≦0.5, and further preferably,0.15≦x≦0.5)  (Formula 4)

Next, a plurality of chip-shaped casts having a size of several mmsquare or less is produced by cutting the magnesium alloy cast.

Then, the chip-shaped casts may be preliminarily molded using acompression means or a means of a plastic processing method, and thusmay be subjected to homogenized heat treatment. At this time, theconditions of the heat treatment are the same as in the secondembodiment.

The chip-shaped cast is generally used as, for example, the raw materialof thixotropic mold.

Note that a mixture of the chip-shaped cast and ceramic particles may bepreliminarily molded using a compression means or a means of a plasticprocessing method, and thus may be subjected to homogenized heattreatment. Furthermore, before the chip-shaped cast is preliminarilymolded, the cast may be additionally subjected to strong distortionprocessing.

Subsequently, plastic processing is performed on the chip-shaped cast,and thus the chip-shaped cast is solidified and molded. Various methodscan be used as the method of performing the plastic processing in thesame way as in the third embodiment. Note that, before the chip-shapedcast is solidified and molded, repetition processing including:mechanical alloying such as a ball mill, a stamp mill, or a high-energyball mill; or bulk mechanical alloying may be added. In addition, afterthe solidification and molding, plastic processing or blastingprocessing may further be added. Furthermore, the magnesium alloy castmay be complexed with intermetallic compound particles, ceramicparticles, fibers or the like, or the cut product may be mixed withceramic particles, fibers or the like.

The plastic-processed product obtained by performing the plasticprocessing as described above has, at room temperature, a crystallinestructure of the hcp structure magnesium phase and the long periodstacking ordered structural phase. At least a part of the long periodstacking ordered structural phase is curved or bent. With regard to theplastic-processed product after the plastic processing is performed,both the Vickers hardness and the yield strength are increased ascompared with the cast before the plastic processing is performed.

The total amount of distortion when the plastic processing is performedon the chip-shaped cast is the same as in the second embodiment.

Heat treatment may be performed on the plastic-processed product afterthe plastic processing is performed on the chip-shaped cast. Theconditions and effects of the heat treatment are the same as in thesecond embodiment.

Also in the present embodiment, the same effects can be obtained as inthe third embodiment.

In addition, in the present embodiment, the chip-shaped cast is producedby cutting the cast, and thus the structure is miniaturized, with theresult that, as compared with the third embodiment, it becomes possibleto produce, for example, the plastic-processed product having highstrength, high ductility and high toughness. Additionally, in themagnesium alloy of the present embodiment, even when the concentrationsof zinc and a rare-earth element are lower than in the magnesium alloyof the first embodiment, it is possible to obtain the properties of highstrength and high toughness.

In addition, the reason why the lower limit of the content of zinc andthe lower limit of the content of the rear-earth element can be as lowas one half that in the third embodiment, respectively, is the same asin the second embodiment.

Furthermore, the magnesium alloy of the present embodiment may containimpurities to the extent of not affecting the properties of the alloy.

Note that the magnesium alloy of the present embodiment may contain, intotal, c atomic % of at least one element selected from a groupconsisting of La, Ce, Pr, Eu and Mm, and c preferably satisfies Formulae6 and 7 below.

0≦c≦2.0  (Formula 6)

0.25≦b+c≦6.0  (Formula 7)

In addition, the magnesium alloy of the present embodiment may contain,in total, c atomic % of at least one element selected from a groupconsisting of Yb, Sm and Nd, and c preferably satisfies Formulae 6 and 7below.

0≦c≦3.0  (Formula 6)

0.25≦b+c≦6.0  (Formula 7)

Additionally, the magnesium alloy of the present embodiment may contain,in total, c atomic % of at least one element selected from a groupconsisting of Yb, Sm and Nd, may contain, in total, d atomic % of atleast one element selected from a group consisting of La, Ce, Pr, Eu,and Mm, and c and d preferably satisfy Formulae 6 to 8 below.

0≦c≦3.0  (Formula 6)

0≦d≦2.0  (Formula 7)

0.25≦b+c+d≦6.0  (Formula 8)

Sixth Embodiment

A method of manufacturing a flame-retardant magnesium alloy according toone aspect of the present invention will be described. Note that, in themethod of manufacturing a flame-retardant magnesium alloy according tothe sixth embodiment, the description of the same parts as in the methodof manufacturing a flame-retardant magnesium alloy according to thefifth embodiment will be omitted as much as possible.

A flame-retardant magnesium alloy which contains a atomic % of Zn, intotal, b atomic % of at least one element selected from a groupconsisting of Gd, Tb, Tm and Lu, and x atomic % of Ca and in which aremaining part is formed of Mg and a, b and x satisfy Formulae 1 to 4below is melted at a temperature of 800° C. or less (preferably, 850° C.or less) and cast. This alloy has an ignition temperature of 800° C. ormore (preferably 850° C. or more) by containing Ca. In this way, amagnesium alloy cast is made. A product cut into a predetermined shapefrom an ingot is used as the magnesium alloy cast.

0.1≦a≦3.0  (Formula 1)

0.25≦b≦5.0  (Formula 2)

2a−3≦b  (Formula 3)

0<x≦0.5 (preferably, 0.1≦x≦0.5, and further preferably,0.15≦x≦0.5)  (Formula 4)

The process of making the above magnesium alloy cast and the subsequentprocesses are the same as those in the fifth embodiment.

Also according to the present embodiment, the same effects can beobtained as in the fifth embodiment.

Note that it is also possible to carry out the first to sixthembodiments described above by appropriately combining the same.

EXAMPLES First Example Production of Sample

The alloy components of samples in a first Example includeMg_(95.75-X)Zn₂Y_(1.9)La_(0.1)Al_(0.25)Ca_(X) (where X=0 to 1.05). Aningot of a magnesium alloy having these alloy components was meltedusing a high frequency melting furnace in the atmosphere, and castmembers each having a shape of φ32×70 mm were produced by being cut fromthe ingot. These cast members were extruded under conditions of atemperature of 350° C., an extrusion ratio of 10 and an extrusion rateof 2.5 mm/second.

(Tensile Test)

In the extrusion member after the extrusion processing, tensile yieldstrength and elongation were measured at room temperature with a tensiletest, and the results thereof are shown in FIG. 1. In FIG. 1, ▪represents the tensile yield strength, and  represents the elongation.

In the extrusion member, tensile yield strength and elongation weremeasured at a temperature of 523K with a tensile test, and the resultsthereof are shown in FIG. 2. In FIG. 2, ▪ represents the tensile yieldstrength, and  represents the elongation.

(Measurement of Ignition Temperature)

The ignition temperature of each of the cast members was measured. Ameasurement method is as follows.

After the ingot of each of the cast members was processed with a lathein the shape of a chip, a chip of 0.5 g having a given size was put intoan electric furnace and the ignition temperature was measured underheating (100K/min).

The results of the measurement performed as described above are shown inFIG. 3.

In FIG. 3, when the content of Ca was 0.15 to 0.6 atomic %, the ignitiontemperature of the magnesium alloy shown was 850° C. or more. In otherwords, the content of Ca is set to more than 0 atomic % and less than0.75 atomic % (preferably set to not less than 0.1 atomic % and lessthan 0.75 atomic %), and thus the ignition temperature of 800° C. ormore can be expected.

On the other hand, the ignition temperature of a composition to which Cais not added, for example, an alloy ofMg_(95.75)Zn₂Y_(1.9)La_(0.1)Al_(0.25) is approximately 775° C., and thisignition temperature is close to 750° C. which is the temperature at thetime of melting and casting of this alloy. Therefore, when this alloy ismelted, it is necessary to use an atmosphere of an inert gas. However,as with the sample of the present Example, when the ignition temperatureis 800° C. or more or 850° C. or more, it becomes possible to performmelting processing even without the use of an inert gas since theignition temperature is sufficiently higher than the melting point ofthe alloy.

It was confirmed from FIGS. 1 and 2 that when the content of Ca exceeds0.5 atomic %, the tensile strength at room temperature and the tensilestrength at a high temperature (523K) are significantly lowered.Therefore, the addition range of Ca having an ignition temperature of800° C. or more or 850° C. or more while having excellent mechanicalproperties of the long period stacking ordered magnesium alloy is morethan 0 atomic % and not more than 0.5 atomic % (preferably, 0.1 to 0.5atomic %) by addition of Ca to the long period stacking orderedmagnesium alloy.

(Crystalline Structure of Extrusion Member)

The structure observation of the extrusion member subjected to theextrusion processing was made by SEM and EDS. The results thereof areshown in FIGS. 4 and 5.

Comparative Example Production of Sample

The alloy components of samples in a Comparative Example includeMg_(96-X)Zn₂Y_(1.9)La_(0.1)Al_(X) (where X=0 to 0.5). An ingot of amagnesium alloy having these alloy components was melted using a highfrequency melting furnace in an atmosphere of Ar, and cast members eachhaving a shape of φ32×70 mm were produced by being cut from the ingot.These cast members were extruded under conditions of a temperature of350° C., an extrusion ratio of 10 and an extrusion rate of 2.5mm/second.

(Tensile Test)

In the extrusion member after the extrusion processing, tensile strengthand elongation were measured at room temperature with a tensile test,and the results thereof are shown in FIG. 6. In FIG. 6, ▪ represents thetensile strength (σ_(UTS)), ▴ represents a yield strength (σ_(0.2)) and represents the elongation (%).

In the extrusion member, tensile yield strength and elongation weremeasured at a temperature of 523K with a tensile test, and the resultsthereof are shown in FIG. 7. In FIG. 7, ▪ represents the tensilestrength (σ_(UTS)), ▴ represents a yield strength (τ_(0.2)) and represents the elongation (%).

It was confirmed from FIG. 7 that when the content of Al exceeds 0.3atomic, the tensile strength at high temperature (523K) is lowered.Therefore, the content of Al is set to less than 0.35 atomic %(preferably set to 0.3 atomic % or less), and thus it is possible tomaintain high strength at high temperature.

(Crystalline Structure of Extrusion Member)

The structure observation of the extrusion member on which the extrusionprocessing was performed was made by SEM and EDS. The results thereofare shown in FIGS. 8 to 12.

Furthermore, in the same method as in the Comparative Example, a sampleof the extrusion member of Mg_(98.4-X)Zn_(X)Y_(1.5)La_(0.1) (whereX=0.25, 0.5, 1.0, 1.5 and 2.0) was produced, and the structureobservation thereof was performed. The results thereof are shown in FIG.14.

(Creep Test)

A creep test was performed on a sample of the extrusion member. Thealloy components of the samples includeMg_(96-X)Zn₂Y_(1.9)La_(0.1)Al_(X) (where X=0, 0.05, 0.15 and 0.25). Inaddition, in the same method as in the Comparative Example, a sample ofthe extrusion member of the alloy of Mg₉₆Zn₂Y₂ was produced, and a creeptest was performed. The condition of the creep test was 200° C. and 150MPa. The results thereof are shown in FIG. 13.

Second Example Production of Sample

The alloy components of samples in the second Example are as shown inTable 1 and Table 2. An ingot of a magnesium alloy having these alloycomponents was melted using a high frequency melting furnace in theatmosphere, and cast members each having a shape of φ32×70 mm wereproduced by being cut from the ingot. These cast members were extrudedunder conditions of a temperature of 350° C., an extrusion ratio of 10and an extrusion rate of 2.5 mm/second.

(Measurement of Ignition Temperature)

The ignition temperature of each of the cast members was measured. Ameasurement method is as follows.

After the ingot of each of the cast members was processed with a lathein the shape of a chip, a chip of 0.5 g having a given size was put intoan electric furnace and the ignition temperature was measured underheating (100 K/min).

The results of the measurement performed as described above are shown inTable 1.

It was found from Table 1 and Table 2 that it was possible to increasethe ignition temperature of the magnesium alloy by containing Ca.

TABLE 1 Number Alloy Components Ignition temperature ° C. 1Mg_(95.7)Zn₂Gd₂Ca_(0.3) 868 2 Mg_(95.7)Zn₂Tb₂Ca_(0.3) 855 3Mg_(95.9)Zn₂Gd₂Ca_(0.1) 833 4 Mg_(95.9)Zn₂Tb₂Ca_(0.1) 841 5Mg_(95.45)Zn₂Gd₂Al_(0.25)Ca_(0.3) 867 6Mg_(95.45)Zn₂Tb₂Al_(0.25)Ca_(0.3) 861 7 Mg_(95.6)Zn₂Gd₂La_(0.1)Ca_(0.3)834 8 Mg_(95.6)Zn₂Tb₂La_(0.1)Ca_(0.3) 845 9Mg_(95.6)Zn₂Gd₂Ce_(0.1)Ca_(0.3) 855 10 Mg_(95.6)Zn₂Gd₂Yb_(0.1)Ca_(0.3)836 11 Mg_(95.6)Zn₂Gd₂Nd_(0.1)Ca_(0.3) 851 12Mg_(95.5)Zn₂Gd₂Sm_(0.1)La_(0.1)Ca_(0.3) 822 13Mg_(95.5)Zn₂Tb₂Yb_(0.1)La_(0.1)Ca_(0.3) 825 14Mg_(95.35)Zn₂Gd₂Al_(0.25)Ti_(0.1)Ca_(0.3) 863 15Mg_(95.5)Zn₂Gd₂Ce_(0.1)Ti_(0.1)Ca_(0.3) 821

TABLE 2 Number Alloy Components Ignition temperature ° C. 1Mg_(95.7)Zn₂Y₂Ca_(0.3) 832 2 Mg_(95.55)Zn₂Y_(1.9)Al_(0.25)Ca_(0.3) 870 3Mg_(95.7)Zn₂Y_(1.9)Sm_(0.1)Ca_(0.3) 832 4Mg_(95.7)Zn₂Y_(1.9)Nd_(0.1)Ca_(0.3) 829 5Mg_(95.7)Zn₂Y_(1.9)La_(0.1)Ca_(0.3) 878 6Mg_(95.7)Zn₂Y_(1.9)Ce_(0.1)Ca_(0.3) 869 7Mg_(95.7)Zn₂Y_(1.9)Gd_(0.1)Ca_(0.3) 862 8Mg_(95.6)Zn₂Y_(1.9)Gd_(0.1)Sm_(0.1)Ca_(0.3) 842 9Mg_(95.6)Zn₂Y_(1.9)La_(0.1)Nd_(0.1)Ca_(0.3) 850 10Mg_(95.6)Zn₂Y_(1.9)Ce_(0.1)Yb_(0.1)Ca_(0.3) 891 11Mg_(95.45)Zn₂Y_(1.9)La_(0.1)Al_(0.25)Ca_(0.3) 936 12Mg_(95.6)Zn₂Y_(1.9)La_(0.1)Al_(0.25)Ca_(0.15) 865 13Mg_(95.3)Zn₂Y_(1.9)La_(0.1)Al_(0.25)Ca_(0.45) 906 14Mg_(95.4)Zn₂Y_(1.9)La_(0.1)Nd_(0.1)Zr_(0.2)Ca_(0.3) 853 15Mg_(95.45)Zn₂Y_(1.9)Gd_(0.1)Al_(0.25)Ca_(0.3) 910

1. A method of manufacturing a flame-retardant magnesium alloycomprising a step of melting a flame-retardant magnesium alloy whichcontains a atomic % of Zn, x atomic % of Ca, in total, b atomic % of atleast one element selected from a group consisting of Gd, Tb, Tm and Lu,and a residue of Mg, wherein a, b and x satisfy formulae 1 to 4 below,0.1≦a≦5.0  (Formula 1)0.25≦b≦5.0  (Formula 2)0.5a−0.5≦b  (Formula 3)0<x≦0.5.  (Formula 4)
 2. A method of manufacturing a flame-retardantmagnesium alloy comprising a step of melting a flame-retardant magnesiumalloy which contains a atomic % of Zn, x atomic % of Ca, in total, batomic % of at least one element selected from a group consisting of Gd,Tb, Tm and Lu, and a residue of Mg, wherein a, b and x satisfy formulae1 to 4 below,0.1≦a≦3.0  (Formula 1)0.25≦b≦5.0  (Formula 2)2a−3≦b  (Formula 3)0<x≦0.5.  (Formula 4)
 3. The method of manufacturing a flame-retardantmagnesium alloy according to claim 1, wherein a flame-retardantmagnesium alloy in which said a and b satisfy Formulae 1′ and 2′ belowis melted,0.2≦a≦5.0  (Formula 1′)0.5≦b≦5.0.  (Formula 2′)
 4. The method of manufacturing aflame-retardant magnesium alloy according to claim 2, wherein aflame-retardant magnesium alloy in which said a and b satisfy Formulae1′ and 2′ below is melted,0.2≦a≦3.0  (Formula 1′)0.5≦b≦5.0.  (Formula 2′)
 5. The method of manufacturing aflame-retardant magnesium alloy according to claim 1, wherein saidflame-retardant magnesium alloy has an ignition temperature of 800° C.or more.
 6. The method of manufacturing a flame-retardant magnesiumalloy according to claim 1, wherein said flame-retardant magnesium alloyis melted at a temperature of 800° C. or less.
 7. The method ofmanufacturing a flame-retardant magnesium alloy according to claim 1,wherein said flame-retardant magnesium alloy is melted, and then themelted flame-retardant magnesium alloy is cast.
 8. The method ofmanufacturing a flame-retardant magnesium alloy according to claim 7,wherein a cooling rate in casting said flame-retardant magnesium alloyis 1000K/second or less.
 9. The method of manufacturing aflame-retardant magnesium alloy according to claim 1, wherein saidflame-retardant magnesium alloy contains y atomic % of Al, and ysatisfies Formula 5 below,0<y≦0.5.  (Formula 5)
 10. The method of manufacturing a flame-retardantmagnesium alloy according to claim 3, wherein said flame-retardantmagnesium alloy contains, in total, c atomic % of at least one elementselected from a group consisting of La, Ce, Pr, Eu and Mm, and csatisfies Formula 6 and Formula 7 below,0≦c≦2.0  (Formula 6)0.5≦b+c≦6.0.  (Formula 7)
 11. The method of manufacturing aflame-retardant magnesium alloy according to claim 1, wherein saidflame-retardant magnesium alloy contains, in total, c atomic % of atleast one element selected from a group consisting of La, Ce, Pr, Eu andMm, and c satisfies Formula 6 and Formula 7 below,0≦c≦2.0  (Formula 6)0.25≦b+c≦6.0.  (Formula 7)
 12. The method of manufacturing aflame-retardant magnesium alloy according to claim 3, wherein saidflame-retardant magnesium alloy contains, in total, c atomic % of atleast one element selected from a group consisting of Yb, Sm and Nd, andc satisfies Formula 6 and Formula 7 below,0≦c≦3.0  (Formula 6)0.5≦b+c≦6.0.  (Formula 7)
 13. The method of manufacturing aflame-retardant magnesium alloy according to claim 1, wherein saidflame-retardant magnesium alloy contains, in total, c atomic % of atleast one element selected from a group consisting of Yb, Sm and Nd, andc satisfies Formula 6 and Formula 7 below,0≦c≦3.0  (Formula 6)0.25≦b+c≦6.0.  (Formula 7)
 14. The method of manufacturing aflame-retardant magnesium alloy according to claim 3, wherein saidflame-retardant magnesium alloy contains, in total, c atomic % of atleast one element selected from a group consisting of Yb, Sm and Nd,contains, in total, d atomic % of at least one element selected from agroup consisting of La, Ce, Pr, Eu and Mm, and c and d satisfy Formulae6 to 8 below,0≦c≦3.0  (Formula 6)0≦d≦2.0  (Formula 7)0.5≦b+c+d≦6.0.  (Formula 8)
 15. The method of manufacturing aflame-retardant magnesium alloy according to claim 1, wherein saidflame-retardant magnesium alloy contains, in total, c atomic % of atleast one element selected from a group consisting of Yb, Sm and Nd,contains, in total, d atomic % of at least one element selected from agroup consisting of La, Ce, Pr, Eu and Mm, and c and d satisfy Formulae6 to 8 below,0≦c≦3.0  (Formula 6)0≦d≦2.0  (Formula 7)0.25≦b+c+d≦6.0.  (Formula 8)
 16. The method of manufacturing aflame-retardant magnesium alloy according to claim 1, wherein saidflame-retardant magnesium alloy contains, in total, more than 0 atomic %and not more than 2.5 atomic % of at least one element selected from agroup consisting of Th, Si, Mn, Zr, Ti, Hf, Nb, Ag, Sr, Sc, B, C, Sn,Au, Ba, Ge, Bi, Ga, In, Ir, Li, Pd, Sb and V.
 17. A flame-retardantmagnesium alloy comprising a atomic % of Zn, x atomic % of Ca, in total,b atomic % of at least one element selected from a group consisting ofGd, Tb, Tm and Lu, and a residue of Mg, wherein a, b and x satisfyFormulae 1 to 4 below, and said alloy comprises a crystalline structurehaving a long period stacking ordered structural phase,0.1≦a≦5.0  (Formula 1)0.25≦b≦5.0  (Formula 2)0.5a−0.5≦b  (Formula 3)0<x≦0.5.  (Formula 4)
 18. A flame-retardant magnesium alloy comprising aatomic % of Zn, x atomic % of Ca, in total, b atomic % of at least oneelement selected from a group consisting of Gd, Tb, Tm and Lu, and aresidue of Mg, wherein a, b and x satisfy Formulae 1 to 4 below, andsaid alloy comprises a crystalline structure having a long periodstacking ordered structural phase,0.1≦a≦3.0  (Formula 1)0.25≦b≦5.0  (Formula 2)2a−3≦b  (Formula 3)0<x≦0.5.  (Formula 4)
 19. The flame-retardant magnesium alloy accordingto claim 17, wherein a flame-retardant magnesium alloy in which said aand b satisfy Formulae 1′ and 2′ below is melted,0.2≦a≦5.0  (Formula 1′)0.5≦b≦5.0.  (Formula 2′)
 20. The flame-retardant magnesium alloyaccording to claim 18, wherein a flame-retardant magnesium alloy inwhich said a and b satisfy Formulae 1′ and 2′ below is melted,0.2≦a≦3.0  (Formula 1′)0.5≦b≦5.0.  (Formula 2′)
 21. The flame-retardant magnesium alloyaccording to claim 17, wherein said alloy has an ignition temperature of800° C. or more.
 22. The flame-retardant magnesium alloy according toclaim 17, wherein said alloy contains y atomic % of Al, and y satisfiesFormula 5 below,0<y≦0.5.  (Formula 5)
 23. The flame-retardant magnesium alloy accordingto claim 19, wherein said alloy contains, in total, c atomic % of atleast one element selected from a group consisting of La, Ce, Pr, Eu andMm, and c satisfies Formula 6 and Formula 7 below,0≦c≦2.0  (Formula 6)0.5≦b+c≦6.0.  (Formula 7)
 24. The flame-retardant magnesium alloyaccording to claim 17, wherein said alloy contains, in total, c atomic %of at least one element selected from a group consisting of La, Ce, Pr,Eu and Mm, and c satisfies Formula 6 and Formula 7 below,0≦c≦2.0  (Formula 6)0.25≦b+c≦6.0.  (Formula 7)
 25. The flame-retardant magnesium alloyaccording to claim 19, wherein said alloy contains, in total, c atomic %of at least one element selected from a group consisting of Yb, Sm andNd, and c satisfies Formula 6 and Formula 7 below,0≦c≦3.0  (Formula 6)0.5≦b+c≦6.0.  (Formula 7)
 26. The flame-retardant magnesium alloyaccording to claim 17, wherein said alloy contains, in total, c atomic %of at least one element selected from a group consisting of Yb, Sm andNd, and c satisfies Formula 6 and Formula 7 below,0≦c≦3.0  (Formula 6)0.25≦b+c≦6.0.  (Formula 7)
 27. The flame-retardant magnesium alloyaccording to claim 19, wherein said alloy contains, in total, c atomic %of at least one element selected from a group consisting of Yb, Sm andNd, contains, in total, d atomic % of at least one element selected froma group consisting of La, Ce, Pr, Eu and Mm, and c and d satisfyFormulae 6 to 8 below,0≦c≦3.0  (Formula 6)0≦d≦2.0  (Formula 7)0.5≦b+c+d≦6.0.  (Formula 8)
 28. The flame-retardant magnesium alloyaccording to claim 17, wherein said alloy contains, in total, c atomic %of at least one element selected from a group consisting of Yb, Sm andNd, contains, in total, d atomic % of at least one element selected froma group consisting of La, Ce, Pr, Eu and Mm, and c and d satisfyFormulae 6 to 8 below,0≦c≦3.0  (Formula 6)0≦d≦2.0  (Formula 7)0.25≦b+c+d≦6.0.  (Formula 8)
 29. The flame-retardant magnesium alloyaccording to claim 17, wherein said alloy contains, in total, more than0 atomic % and not more than 2.5 atomic % of at least one elementselected from a group consisting of Th, Si, Mn, Zr, Ti, Hf, Nb, Ag, Sr,Sc, B, C, Sn, Au, Ba, Ge, Bi, Ga, In, Ir, Li, Pd, Sb and V.
 30. Theflame-retardant magnesium alloy according to claim 17, wherein saidalloy is a cast.